High Quality Content by WIKIPEDIA articles! The heavy machine gun is a larger class of machine gun generally recognized to refer to two separate stages of machine gun development. The term was originally used to refer to the early generation of machine guns which came into widespread use in World War I. These fired the standard (~.30 or 7.62mm) rifle cartridge but featured heavy construction, elaborate mountings, and water cooling mechanisms that enabled heavy and sustained defensive fire with excellent accuracy, but with the cost of being too cumbersome to move quickly. Thus, in this sense, the "heavy" aspect of the weapon referred to the weapon's bulk and ability to sustain fire, not the cartridge caliber. This class of weapons is best exemplified by the Maxim gun, invented by American Hiram Maxim. The Maxim was the most ubiquitous machine gun of World War I, regional variants of which were fielded simultaneously by three separate warring nations (Germany with the MG08 in 8mm Mauser, Britain with the Vickers in .303 British, and Russia with the Pulemyot M1910 in 7.62x54R).